Field of the Invention
The invention relates to a sheet-fed rotary printing machine having a plurality of units arranged in tandem, at least one unit being an offset printing unit or a varnishing unit, which includes a first cylinder transporting the sheet and a second cylinder opposite the first cylinder, the first and the second cylinder being mounted so that they are disengageable from one another. The invention also relates to a method of transporting sheets through at least one of the units of the printing machine.
In multicolor printing machines of the foregoing type, downline of which a varnishing unit may be arranged, the paper sheets are often printed with color only in the first printing units. Following printing units or the varnishing unit run "empty" at the same time if this is required by the relevant print job, specifically if either the maximum number of colors for which the machine is equipped are not needed, or if no application of varnish is desired by the customer.
When "running empty" in this manner, the previously freshly printed sheets are conveyed through the succeeding printing unit or units, while the rubber blanket cylinder of the offset printing unit, for example, or the varnishing blanket cylinder of the varnishing unit are disengaged from the associated impression cylinder, i.e, the sheets are gripped at the leading end thereof by grippers on the respective impression cylinder and, in contact wit h the latter, are transported through the small nip which is then produced between the respective impression cylinder and the rubber blanket and varnishing blanket cylinder, respectively.
Because of the high machine speeds, however, the sheets lift off the respective impression cylinder, and the freshly printed side of the sheets touches the rubber blanket or varnishing blanket, with the result that the ink applied to the sheets becomes smeared. This can occur even at relatively low speeds if the printing material has a given stiffness. This is because heavy paper grades have a greater tendency to lift off the impression cylinder because of the centrifugal forces, as do stiff boards as well as a result of the attempt thereof to assume the stretched-out position, and become smeared on the rubber blanket cylinder in the narrow nip of about 5 mm. between the impression cylinder and the rubber blanket cylinder.
In order to avoid this problem, it has been proposed, for example in the published German Patent Document DE 43 18 777 C2, to hold down the freshly printing sheets with blown air as they pass through the printing nip. German Patents DE 689 632 and DE 44 43 493 treat in general terms the problem of flexurally stiff materials lifting off the impression cylinder, and propose the provision of cones or cylindrical rollers which act mechanically upon the edge of the sheet to be printed, so that the sheets to be printed are held down thereby. However, such cones or cylindrical rollers cannot readily be arranged at the critical location, namely the narrow nip between the impression cylinder and the rubber blanket or varnishing blanket cylinder, the nip, after the rubber blanket or varnishing blanket has been disengaged or brought out of contact and removed, being about 5 mm. wide.